Protective circuits for electron tubes



Dec. 18, 1962 G. F. BAVOR ETAL 3,069,548

PROTECTIVE CIRCUITS FOR ELECTRON TUBES Filed Dec. 17, 1958 l8 2O\ 6 PULSE FORMING NESQFSIVE 4 l3 AND cIRcuIT CONTROL cIRcuIT 2| i HIGH 22* VOLTAGE DETECTING OVERLOAD MEANS GENERATOR PROTECTIVE FIGJ CIRCUIT l2 l5 II I NEGATIVE l6 LI4 PULSE FORMING HAS 7 AND cIRcuIT 25 CONTROL CIRCUIT HGH 26 r VOLTAGE GENERATOR FIG. 2 l5 l3 I2 IO NEGATIVE H BIAS I? CIRCUIT 25 INVENTORS I GORDON F. BAVOR 75: BY HOWARD D. DOOLITTLE WERNER BRUNHART AGENT dfihhfidd Patented Klee. 18, 198.2

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3,tl69,54t5 PRGTECTIVE UIRQUE'ES FOR ELECTRGN TUBES Gordon IF. Bayer, Norwaih, Howard D. lficoiittle, Stam= ford, and Werner runhart, Norwalh, (Conn, assignors, by mesne assignments, to The Machlett Laboratories, incorporated, Springdale, Conn, a corporation of Cons necticnt Filed Dec. 17, 1958, Ser. No. 781,111 14 Claims. (Cl. fill-4W3) This invention relates to improvements in electron tube protective systems and particularly to systems for protecting electron tubes from damage by gas ionization or flash arcs which produce abnormally high currents in a series circuit which includes an electron tube and an energy storage capacitor.

In United States Patent No. 2,879,404, Rogers et al., which is assigned to the same assignee as the present invention, there is described and claimed a system for applying electrical pulses to an energy responsive device, such as an X-ray tube, for ultra-short time intervals from energy storage capaciors connected with the energy responsive device through an electronic switching tube.

In such a system permanent damage to either the energy responsive device or the switching tube, or both, has been occasioned by flash arcs in the energy responsive device which lead to excessive voltage on the electronic switching tube and resultant excessive anode dissipation. This causes excessive energy emission from the storage capacitors, resulting in the damage to the device.

In the aforementioned patent, the means for initiating action of the switching tube is a pulse-forming circuit which is coupled to the grid of the high-tension switching tube by a coupling transformer. In accordance with this invention, the secondary of the coupling transformer is short-circuited by low impedance means which is connected with current detecting means in series with the cathode of the switching tube, which detecting means functions as a signal source. Currents above normal in the main circuit are detected by the detecting means which then transmits a signal to the low impedance means, which in turn reduces the normal grid-driving pulse for the switching tube to a very low value, allowing negative grid bias of the tube to place the tube in a cutotf condition. This removes voltage from the energy-responsive device, disrupts the flash arc condition, and prevents damage to the device.

Detection of above-normal currents and operation of the protective circuit occur with such speed that no permanent damage results to either the switching tube or the energy-rcsponsive device.

Accordingly, it is a primary object of this invention to provide novel means for preventing damage to electron discharge devices caused by abnormally high power levels.

Another object is to provide, in a circuit embodying an energy-responsive device and a switching tube operable when conductive to initiate operation thereof, means for detecting abnormal currents in the circuit and responsive thereto for rendering the switching tube nonconductive to remove voltages before damage occurs to the elements embodied in the circuit.

Qther objects and advantages of this invention will become apparent from the following description taken in connection with the accompanying drawings wherein:

FIG. 1 is a schematic and block diagram illustrating the protective circuit of this invention applied to a main or primary circuit in which are located the electronic components being protected;

FIG. 2 is a diagram similar to FIG. 1 illustrating a s eciiic type of protective means and detector; and

FIG. 3 is a diagram similar to FIG. 2 showing a modification thereof.

Referring to FIG. 1, there is shown an energy responsive device lltl, which is indicated in the drawing and hereinafter described as an X-ray tube. X-ray tube in may be of any conventional type embodying a cathode l1 and an anode 12 within the usual vacuum envelope (not shown). A high-voltage generator 13 is utilized to charge two energy storage capacitors 14-45 in a balanced circuit as the primary source of energy for X-ray tube 10. A high-voltage electronic switching tube 16 is in series with the positive side of the high voltage generator circuit to the anode 12 of the X-ray tube It To activate the X-ray tube 10, a signal in the form of a pulse is applied to the grid 17 of the switching tube to from suitable pulse-forming and control circuits 18.

The pulse-forming circuit 38 serves, when the device is operated, to apply positive driving potential to the grid 17 through special circuitry which is fully described in aforementioned Patent No. 2,879,404 and which is shown schematically in the drawing appended hereto as negative bias circuit 1253 which normally provides a negative potential sufficient to maintain the switching tube It) in a cutofi condition.

in operation of the device, the energy storage capacitors i i-15 are energized by generator 13 in advance of each pulse-forming cycle. When a pulse is formed and transmitted to the negative bias circuit 2d, the negative bias normally on grid I7 is overcome and the grid is driven positively an appropriate amount to cause conduction of switching tube This closes the circuit through the X-ray tube to, causing the tube in to operate.

It was found, however, that at times, gas ionization in the X-ray tube it? produced flash arcs, thus producing abnor ally high currents in the circuit and often damaging either or both of the tubes in and in. To overcome this problem in accordance with the present invention, we have provided means for quickly detecting such abnormal currents and for causing the negative grid bias to be returned to the swiiching tube in, thereby opening the circuit to X-ray tube 1b and completely stopping the how of current thereto.

This is accomplished as shown in FIG. 1 by shortcircuiting the secondary of a pulse transformer in the pulse-forming circuit 13 with an overload protective means which has an impedance which is low compared to the input impedance of the grid circut of the switching tube 1 .6. This low impedance protective means may be any device such as a tbyratron, a vacuum tube, a multivibrator circuit, a vacuum switch or other device having the required impedance characteristics. Connected into the main circuit is a current detecting means 22 which is also connected to the protective means 21. Detecting means 22 may be a resistance or an inductance such as an air-core or an iron-core choke, and functions as signal source for triggering the protective means upon detection of excessive currents in the system.

Referring to FIG. 2, the overload protective means is shown as a thyratron tube 2.3 and the current detector is shown as a resistance 24 connected in series with the X-ray tube. For convenience the resistance 24 is actually shown inserted in the sin circuit between the anode 12 of X-ray tube and the cathode 25' of switching tube 16. The grid as of thyratron 2.): is connected between the resistance 24 and grid 37 of switching tube 16 through a current limiting resistor 27.

In the operation of the device as now described, normal current through the switching tube 16, X-ray tube lit? and resistance 24 will not develop sumcient voltage to trigge the thyratron 23. However, a flash arc in the X-ray tube will develop a spike of rapidly rising voltage. When the spike has assumed an amplitude greater than the normal current, it will fire the thyratron. The thyratron will function as a low impedance or short circut across the 34 secondary of the pulse transformer 12? and, being of lower impedance than the input impedance of the grid circuit of tube 16, thus reduces the grid-driving pulse for switching tube 16 to a very low value, allowing the negative b ..s to recover and place the switching tube in non-conducting condition.

This completely removes voltage from the X-ray tube, permitting the gas to deio-nize, thus discontinuing the flash arc condition and thereby preventing damage to the tubes. The detection of the excess current is accomplished etore suflicient time has expired to the tubes, usually within a very few microseconds.

In FIG. 3 there is shown another type of current detecting means for triggering the overload protective circuit. In this modification the detec means comprises a resistance and capacitor combination 28-23 which is series connected between the cathode 25 of switching tube and ground. A signal for triggering the overload protective device, in this case thyratron 23, may be obtained from the cable 35 which connects the cathode 25 of tube 16 with the anode 12 of X-ray tube 1d. The resistancecapacitor combination 2829 is designed to have a time constant such that the rate of change of potential on the detecting circuit capacitor 29 is slower than the potential change of the capacity formed by a distributed circuit capacitance to ground from the cable Fail.

Thus, when excess current is built up in the series circuit embodying switching tube 16 and X-ray tube 19, the change in potential on capacitor 29 with respect to the potential on the distributed capacity in cable 31 is such that the potential on capacitor 2%, which is connected to the grid 26 of thyratron 23, is maintained more positive with respect to the cathode of the thyratron. The potential difference between grid as and the adjacent cathode causes the thyratron to fire and to function in the same manner as the thyratron 23 in FIG. 2 to render the switching tube 16 nonconductive to completely remove voltage from X-ray tube lit.

From the foregoing description it will be apparent that we have provided a circuit embodying a grid-controlled switching tube and energy-responsive device in series with energy storage means, means for applying pulses to the grid of the switching tube for overcoming negative bias thereon, and novel means for restoring the negative bias when the energy-responsive device becomes faulty.

It will be apparent, however, that various modifications may be made by those skilled in the art, such as by using a multivibrator device instead of the thyratron or a coil instead of the resistor 22, without departing from the spirit of the invention as expressed in the accompanying claims.

We claim:

1. An electronic control system comprising an energyresponsive device and a grid-type switching device connected in a series circuit to a first source of power, a second source of power connected to the grid of said switching device, said switching device being operable when power is supplied to the grid from said second source of power to cause flow of current through the energy-responsive device and the switching device from the first source of power, and means responsive to excessive currents in said series circuit for interrupting the flow of current from said second source of power to the grid of the switching device.

2. An electronic control system comprising a grid-controlled switching tube and an energy-responsive device connected in a series circuit to a source of power, signal forming means connected to the grid of the switching tube for transmitting a signal thereto for rendering the switching tube conductive and to cause flow of current to the energy-responsive device from said source of power, means for protecting the energy-responsive device from damage by excess current comprising detection means connected to said series circuit for detecting excess current in the series circuit, and low impedance means connected across the signal-forming means a d operable upon detection by the detection means of excess current in the series circuit to reduce the signal from the s l-forming means to render the switching tube noncondtcte and completely remove voltage from the ener y rescpcnsive device.

3. An electronic control system comprising a grid-controlled switching tube and an energy-responsive device connected in a series circuit to a. source of power, signalforming means connected to the grid of the switching tube for transmitting a signal thereto for rendering the switching tube conductive and to cause How of current to the energy-responsive device, from said source of power, means for protecting the energy-responsive device from damage by excess current comprising detection means in series with and between the cathode of the switching tube and anode of the energy-responsive device for detec g excess current in the series circuit, and low impedance means connected across the signal-forming means and operable by means which is connected to the series circuit between the switching tube and detection means upon detection by the etection means of excess current in the series circuit to reduce the signal from the signalforming means to render the switching tube nonconductive and remove volta e from the energy-responsive device.

4. An electronic control system co uprising a grid-controlled switching tube and an energ -rcsponsive device connected in a series circuit to a source of power, signalforrning means connected to the grid of the switching tube for transmitting a signal thereto for rendering the switching tube conductive and to cause flow of current to the energy-responsive device, from said source of power, means for protecting the energy-responsive device from damage by excess current comprising a resistance in series with the switching tube and energy-responsive device fordetecting excess current in the series circuit, and a low impedance grid-controlled vacuum tube connected across the signal-forming means and having its grid connected to the series circuit between the switching tube and resistance, said grid of the vacuum tube being operable to fire the vacuum tube upon detection of excess current in the series circuit by said resistance to reduce the signal from the signal-torming means to the grid of the switching tube and render the switching tube nonconductive and remove voltage from the energy responsive device.

5. Protective means substantially as set forth in claim 4 wherein the grid-controlled vacuum tube is a thyratron.

6. An electronic system for controlling operation of an X-ray tube comprising a grid-controlled switching tube connected in a series circuit with the X-ray tube to a source of power, signal-forming means connected to the grid of the switching tube for transmitting a signal thereto for rendering the switching tube conductive and to cause flow of current to the X-ray tube from said source of power, means for protecting the X-ray tube from damage by excess current comprising detection means connected to said series circuit for detecting excess current in the series circuit, and low impedance means connected across the signal-forming means and operable upon detection of excess current in the series circuit by the detection means to reduce the signal from the signal-forming means to render the switching tube nonconductive and remove voltage from the X-ray tube.

7. An electronic system for controlling operation of an V '"tyt lcc a controhed switching tube connected in a series circuit with the X-ray tube to a source of power, signal-forming means connected to the grid of the switching tube for transmitting a signal thereto for rendering the switching tube conductive and to cause flow of current to the X-ray tube from said source of power, means for protectin the X-ray tube from damage by excess current comprising a resistance in series with and betwee the switching tube and X-ray tube for detecting excess current in the series circuit, and low impedance means connected across the signal-forming means and operable by means which is connected to the series circuit between the switching tube and resistance upon detection of excess current in the series circuit to reduce the signal from the signal-forming means to render the switching tube nonconductive and remove voltage from the X-ray tube.

8. An electronic system for controlling operation of an X-ray tube and comprising a grid-controlled switching tube connected in a series circuit with the X-ray tube to a source of power, signal-forming means connected to the grid of the switching tube for transmitting a signal thereto for rendering the switching tube conductive and to cause flow of current to the X-ray tube from said source of power, means for protecting the X-ray tube from damage by excess current comprising a resistance in series with and between the switching tube and X-ray tube for detecting excess current in the series circuit, and a low impedance grid-controlled vacuum tube connected across the signal-forming means and having its grid connected to the series circuit between the switching tube and resistance, said grid being operable to fire the vacuum tube upon detection of excess current in the series circuit by said resistance to reduce the signal from the signalforming means to the grid of the switching tube and render the switching tube nonconductive and remove voltage from the X-ray tube.

9. Protective means for an X-ray tube substantially as set forth in claim 8 wherein the grid-controlled vacuum tube is a thyratron.

10. An electronic control system for controlling operation of an X-ray tube and comprising a grid-controlled switching tube connected in a series circuit with the X-ray tube, energy storage means in parallel with the X-ray tube and switching tube for energizing the X-ray tube upon operation of the switching tube, a negative bias circuit connected to the grid of the switching tube for main taining the switching tube in a nonconductive condition, pulse-forming means including a transformer having its secondary connected to the negative bias circuit for transmitting a pulse to the grid of the switching tube and causing operation of the switching tube by overcoming said negative bias on the grid, a protective circuit for quickly detecting excess current in said series circuit and operable in response thereto to discontinue operation of the switching tube to remove voltage from the X-ray tube comprising detection means in series with and between the switching tube and X-ray tube, and low impedance means connected across the secondary of the transformer and operable by means which is connected between the switching tube and detection means, said low impedance means being operable upon detection by said detection means of currents in the series circuit which are higher than a predetermined value to reduce the pulse from the transformer to -a low enough value that the negative bias circuit will place the switching tube in a cut-off condition and remove voltage from the X-ray tube.

11. An electronic control system for controlling operation of an Xray tube and comprising a grid-controlled switching tube connected in a series circuit with the X-ray tube, energy storage means in parallel with the X-ray tube and switching tube for energizing the X-ray tube upon operation of the switching tube, a negative bias circuit connected to the grid of the switching tube for maintaining the switching tube in a nonconductive condition, pulseforming means including a transformer having its secondary connected to the negative bias circuit for transmitting a pulse to the grid of the switching tube and causing operation of the switching tube by overcoming said negative bias on the grid, a protective circuit for quickly detecting excess current in said series circuit and operable in response thereto to discontinue operation of the switching tube to remove voltage from the X-ray tube comprising a resistance in series with and between the switching tube and X-ray tube, and a low impedance grid-controlled vacuum tube having its grid electrode connected between the switching tube and resistance, said grid being operable to fire the vacuum tube upon detection by said resistance of current in the series circuit higher than a predetermined value, the vacuum tube being connected across the secondary of the transformer and operable when fired to reduce the pulse from the transformer to a low enough value that the negative bias circuit will place the switching tube in a cut-off condition and remove voltage from the X-ray tube.

12. A protective circuit substantially as set forth in claim 11 wherein the vacuum tube is a. thyratron.

13. An electronic control system comprising a gridcontrolled switching tube and an energy-responsive device connected in a series circuit to a source of power, a signal-forming means connected to the grid of the switching tube for transmitting a signal thereto from a second source of power for rendering the switching tube conductive to operate the energy-responsive device, means for protecting the energy-responsive device from damage by excess current comprising a resistance and capacitor combination which is series connected to the series circuit between the switching tube and energyresponsive device for detecting excess current in the series circuit, and a low impedance overload protective device connected across the signahforming means and also connected to the junction between the resistance and capacitor of said combination, and operable upon detection of excess current by said combination to reduce the signal from the signal-forming means to render the switching tube nonconductive and remove voltage from the energyresponsive device.

14. A system substantially as set forth in claim 13 wherein the resistance-capacitor combination has a time constant such that the rate of change of potentials on the capacitor is slower than the potential change of the capacity formed by a distributed circuit capacitance to ground from the connection between the switching tube and energy-responsive device.

References Iited in the file of this patent UNITED STATES PATENTS 2,222,536 Kuntke Nov. 19, 1940 2,319,378 Weisglass May 18, 1943 2,571,027 Garner Oct. 9, 1951 2,572,258 Goldfield Oct. 23, 1951 2,823,319 Vossberg Feb. 11, 1958 2,829,272 Ball Apr. 1, 1958 2,879,404 Rogers et a1 Mar. 24, 1959 2,936,376 Hoekstra May 10, 1960 1 FOREIGN PATENTS 592,359 Great Britain Sept. 16, 1947 

